Modular wireless test architecture and method

ABSTRACT

A modular test chassis for use in testing wireless devices includes a backplane and a channel emulation module coupled to the backplane. The channel emulation module comprises circuitry for emulating the effects of a dynamic physical environment (including air, interfering signals, interfering structures, movement, etc.) on signals in the transmission channel shared by the first and second device. Different channel emulation modules may be included in the test system depending upon the protocol, network topology or capability under test. A test module may be provided to generate traffic at multiple interfaces of SISO or MIMO DUTs to enable thorough testing of device and system behavior in the presence of emulated network traffic and fault conditions. A latency measurement system and method applies timestamps frames as they are transmit and received at the test module for improved latency measurement accuracy.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to provisionalpatent application Ser. No. 60/670,522 filed Apr. 12, 2005 by Mlinarskyand Wright, incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to the field of test equipment, andmore particularly to an improved architecture for use in testingwireless devices.

BACKGROUND OF THE INVENTION

New wireless technology is being developed and deployed to providesupport for voice and multimedia services in both residential andenterprise environments. Wireless Local Area Network (“WLAN”) devices,for example, are being developed in conjunction with IEEE 802.11standards to support packetized voice communications such as Voice overInternet Protocol (“VoIP”). There are technological hurdles that must beovercome in order to support voice and multimedia on WLANs because thetechnology was initially designed to support simple data communications.In particular, voice and multimedia applications can be more sensitiveto jitter, delay and packet loss than data communications applications.IEEE 802.11 is under development, and continually provides new protocolsand techniques which seek to overcome some of these technologicalhurdles as well as to increase the capacity of a wireless network.

Because the costs associated with developing, purchasing, selling anddeploying a new wireless technology are often quite high, it is commonto conduct testing to mitigate the risk that the technology will fail toperform as planned. However, wireless devices are notoriously difficultto test because they can be affected by ambient sources of interference.Further, the conditions to which a wireless device may be subjected inactual use are so great in number that it is difficult andtime-consuming to create all of those conditions in a test environment.It is known, for example, to simulate some wireless network operationsby manually moving a wireless device through a building in whichwireless access devices, are strategically situated. However, thistechnique is too labor-intensive and imprecise to simulate a widevariety of traffic conditions, distances between access points and ratesof motion in a practical manner. Further, such a manual, open-air testcan be rendered invalid by transient interference from a microwave,RADAR or other RF source. More recently it has become known to simulatea wireless network by enclosing devices in EMI-shielded containers whichare in communication via wired connections. Such a system is disclosedin U.S. Pat. No. 6,724,730 entitled “Test System for Simulating aWireless Environment and Method of Using Same”, by Mlinarsky et al.(herein after the Mlinarsky patent) which is incorporated herein byreference.

FIG. 1 illustrates the prior art architecture of Mlinarsky. A central RFcombiner 110 connected to a plurality of connection nodes 102 viaprogrammable attenuation components 108. A controller console controlsthe programmable attenuation component for the purposes of simulatingspatial positioning of the plurality of connection nodes to facilitateoperational testing of the nodes. As shown in FIG. 1, the RF combinerarrangement enables simulation of movement by the coupled nodes alongthe links of the star topology. While this architecture is effective forsimulating movement within the topology, the simulation ofmulti-dimensional movement is restricted by the available connections.It would therefore be desirable to identify an improved architecturewhich is capable of providing full nodal connectivity to simulatemovement in multiple dimensions.

In addition to identifying an architecture with increased movementsimulation capabilities, it would also desirable to identify a wirelesstest architecture capable of adequately testing the operation ofMultiple Input, Multiple Output (MIMO) devices as defined in IEEE802.11n™. 802.11n is new standard for high-speed wireless local areanetworking, offering throughput greater than 100 Mbps. 802.11n works byutilizing multiple wireless antennas in tandem to transmit and receivedata. The associated term “MIMO” refers to the ability of 802.11n (andother similar technologies) to coordinate multiple simultaneous radiosignals. MIMO increases both the range and throughput of a wirelessnetwork by taking advantage of the distinguishability of signalstransmitted on the same FCC allocated radio channel by different radios.

In general MIMO uses multiple antennas to send multiple distinct signalsacross different spatial paths at the same time, increasing throughput.The radio signals are naturally reflected, absorbed and diffracted asthey propagate through different materials in any enclosed space. Thereflections arrive at a receiver with unpredictable amplitude, time andphase relationships, causing multipath distortion of the originalsignal. High data-rate signals are more susceptible to multipath, whichhas traditionally limited speed and range. The higher the data rate, themore detrimental the multipath distortion is to the signal. MIMO signalprocessing exploits the fact that each different spatial path hasdifferent multipath, by essentially ‘training’ the receivers toassociate the differently distorted received signals with differentradios. This allows MIMO receivers to recover the multiple distincttransmitted signals.

A variety of wireless products will shortly be introduced that operateaccording to the 802.11n protocol. Prior to their introduction, it willbe desirable for vendors to identify methods of testing their devices inorder that they may verify the products' ability to operate according tothe protocol, and also to quantify the capabilities of their product. Itwould therefore be desirable to identify a test architecture which wouldpermit verification of devices operating under the 802.11n protocol.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a system for testing wirelessdevices includes an RF backplane and a channel emulation modulecouplable to the RF backplane. One or more RF combiners may be coupledto the RF backplane, wherein each of the RF combiners includes aplurality of RF connectors adapted to exchange RF signals with a firsttest device. The channel emulation module also includes at least one RFconnector adapted to exchange RF signals with a second test device. Thefirst test device and second test device communicate over selectedtransmission channels. The channel emulation module comprises circuitryfor emulating the effects of a physical environment (including air,interfering signals, interfering structures, etc.) on signals in thetransmission channel. The effects that are emulated by the channelemulation module are referred to herein as ‘channel effects,’ andinclude but are not limited to multipath reflections, delay spread,angle of arrival, power angular spread, angle of departure, antennaspacing, uniform linear array for both TX and RX side, Doppler due tofluorescent light effects, Doppler from moving vehicle, Doppler fromchanging environments, path loss, shadow fading effects and reflectionsin clusters.

The channel emulation module modifies the physical layer of wirelesstransmissions in accordance with the channel effect to be emulated, forexample by increasing signal attenuation to simulate path loss in thetransmission channel coupling the test devices. Different channelemulation modules may be included in the test system depending upon theprotocol, network topology or capability under test. For example,attenuation channel emulation modules may be used to model path loss forwireless systems that use Single Input, Single Output or Multiple Input,Multiple Output transmission channels. A cross-connect channel emulationmodule may be used to emulate multi-dimensional spatial movement of thecoupled test devices for enhanced testing of roaming capabilities.Multipath channel emulation modules may be used to emulate multipathsignal effects for the purposes of testing Multiple Input, MultipleOutput (MIMO) and beam forming technologies. Any combination of thechannel emulation capabilities may be included in different embodimentsof a channel emulator module.

The channel emulator module is thus an interchangeable component of amodular wireless network test architecture that enables testing of awide range of wireless protocols and network topologies. The modularnature of the RF combiners and the channel emulator modules makes iteasy to change test network configuration by simply adding or removingthe combiners and modules on either side of the backplane.

According to another aspect of the invention, a test module is providedfor incorporation in a test environment. The test module includescircuitry for simultaneously generating network traffic over multiplenetwork interfaces. In addition, the test module combines client andaccess point (AP) emulation capabilities with a powerful protocol testautomation environment to enable thorough testing of device and systembehavior in the presence of emulated network traffic and faultconditions.

According to another aspect of the invention, the test module includes alatency measurement method and apparatus, which applies time stamps toframes as they are transmitted and received at various networkinterfaces of the test module. Time stamping frames as they aretransmitted and received reduces nondeterministic delay in themeasurement process, thereby increasing the accuracy of the latencymeasurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art wireless testing environment;

FIG. 2 illustrates a wireless testing environment including a modulartest chassis of the present invention;

FIG. 3 is a diagram of one embodiment of circuitry that may be includedin an attenuation channel emulation module included in test chassis ofFIG. 2;

FIG. 4 is a block diagram of illustrating exemplary components that maybe included in a cross-connect channel emulation module;

FIG. 5 is a block diagram provided to illustrate one embodiment ofcross-connect circuitry that may be included in the cross-connectemulation module of FIG. 4;

FIG. 6 is a block diagram provided to illustrate another embodiment ofcross-connect circuitry that may be included in the cross-connectemulation module of FIG. 4;

FIG. 7 is a diagram provided for the purposes of illustrating multipathsignal propagation;

FIG. 8 is a diagram of a test environment in which the test chassisarchitecture is used to test Multiple Input, Multiple Output (MIMO)devices;

FIG. 9 is a block diagram provided to illustrate several components thatmay be included in a MIMO channel emulation module;

FIG. 10 is a diagram of a first embodiment of channel emulationcircuitry which uses analog circuitry to emulate multipath and otherchannel effects by modifying the signal in the RF domain;

FIG. 11 is a diagram of a second embodiment of channel emulationcircuitry which uses analog circuitry to emulate multipath and otherchannel effects by modifying the signal in the RF domain;

FIG. 12 is a diagram of a third embodiment of channel emulationcircuitry which uses a digital signal processor for emulating multipathand other channel effects;

FIG. 13 is a diagram of a fourth embodiment of the multipath channelemulation circuitry, including a return transmission path havingmultipath effects added to return signals in the IF domain;

FIG. 14 is a diagram illustrating a multipath test environmentconfiguration for use in analyzing roaming behavior of a MIMO device;

FIG. 15 a is a diagram illustrating the coupling of a DUT to astand-alone test module;

FIG. 15 b is a diagram illustrating the coupling of a network of DUTs toa stand-alone test module;

FIG. 16 is a block diagram of several exemplary components that may beincluded in a test module of the present invention;

FIG. 17 is a diagram provided to illustrating an infrastructure testconfiguration using the test module of the present invention;

FIG. 18 is a diagram provided to illustrate the use of a test module forverifying operation of an access point device;

FIG. 19 is a diagram illustrating self-termination circuitry that may beincluded in the RF combiner to automatically provide characteristicimpedance termination when a chassis slot is not populated by a module;and

FIG. 20 is a data flow diagram provided to illustrate a method andsystem for measuring the latency between an RF interface and EthernetInterface of a DUT;

FIG. 21 is a data flow diagram provided to illustrate a method andsystem for measuring the latency between an Ethernet Interface and RFinterface of a DUT; and

FIG. 22 is a data flow diagram provided to illustrate a method andsystem for measuring the latency in an RF input to RF output path of aDUT.

DETAILED DESCRIPTION

An improved test architecture which may be used to test the operation ofwireless devices will now be described. As will be seen, thearchitecture is modular in nature and enables testing of a variety ofnetwork protocols and topologies. For example the architecture may beused to test Single Input, Single Output (SISO) 802.11a, b or g devicesas well as 802.11n Multiple Input, Multiple Output (MIMO) devices. Inaddition to supporting a variety of protocols, the modular nature of thecomponents allows multiple network topologies and multi-dimensionalmovement to be simulated with ease.

Referring now to FIG. 2, one embodiment of a wireless test system 10utilizing the architecture of the present invention is shown. The testsystem 10 is shown to include a backplane matrix 12. The backplanematrix may be housed in an enclosure (not shown), and together thebackplane and enclosure comprise a test chassis. The backplane matrix 12includes slots for coupling one or more RF combiners 13 a-13 c. Each RFcombiner 13 a-13 c is a modular component having a number of RFconnectors for receiving RF signals from coupled RF devices. Anexemplary RF combiner that may be used is described in U.S. Pat. No.6,724,730, incorporated herein by reference and referred to hereinafteras the Mlinarsky patent. In general each RF combiner is a communicationhub which connects all RF devices coupled to the individual combiner.Each RF combiner includes N connectors enabling communication between Ndevices. Although the RF combiner of FIG. 2 includes 8 connectors, itshould be appreciated that the present invention is not limited to anyparticular number of connectors. In addition, although the figures anddescription will reference a system which uses a backplane forsupporting any number of combiners, it is appreciated that equivalentfunctionality may be achieved through a variety of designs and theinvention is not meant to be limited in any manner by the disclosedembodiment.

Another type of module which may be coupled to the test chassis is achannel emulation module 20. Channel emulation module 20 is coupled to acolumn of connectors on a front face 14 of the backplane matrix 12 viabackplane connector column 15. The channel emulation module alsoincludes at least one column of one or more RF ports 21 (shown groupedin a dashed ellipse in FIG. 2) which are used to couple the channelemulation module to an RF device such as device 42. Together the channelemulation module, RF combiners and backplane matrix form a test systemwhich may be used to test one or more wireless devices.

Depending upon the desired test environment, different numbers of RFcombiners and channel emulator modules may be swapped into and out ofthe chassis. As will be shown in several embodiments below, the testenvironment may be modified in an N dimension by adding or removing RFcombiners, and may be modified in an M dimension by addition or removingchannel emulator modules. The modular nature of the architectureincreases the ease of configuration of different test environments,thereby enabling more robust testing of devices to be performed at acommon test chassis.

As will be described in more detail below, the channel emulation modulecomprises circuitry for emulating the effects of a physical environment(air, interference, etc.) on signals in a transmission channel. Theemulation circuitry modifies the physical layer of wirelesstransmissions in accordance with the emulated effect, for example byincreasing or decreasing attenuation or gain to emulate path-loss orfading resulting from movement and changing environmental artifacts.Different channel emulation modules may be swapped into the wirelesstest system 10 depending upon the network protocol, topology orcapability that it is desired to test.

For the purposes of this application, a ‘device under test’ (DUT) is anydevice whose operation and/or performance is being monitored oranalyzed; a System Under Test (SUT) is a wireless system including oneor more DUTs; and a test device is a device included in the test networkwhich is used to exercise or monitor the DUT during a test as part ofthe test bed. The wireless test system of the present invention supportstesting of a variety of RF devices and a variety of RF protocols. Ingeneral, the DUTs' behavior in response to test stimulus, wherein thetest stimulus may be a changing physical environment, is monitored andanalyzed. Because of the inherent sensitivity of wireless devices totheir physical RF environment, and in order to ensure that behavior ofthe DUT is attributable only to the test stimulus, test devices and DUTs(such as devices 32 and 42) are generally isolated from environmentaleffects, and each other, by placement in shielded test chambers (test“heads”), such as chambers 132 and 142. Shielded RF cables 33 and 41 arecoupled to the RF antenna ports of the test devices, and are used tocouple the DUTs and test devices to their respective RF combiner andchannel emulation connectors. Isolating the devices and their RF signalsusing test heads and cables in this manner helps to ensure that thebehavior of the devices is attributable only to the test stimuli, theapplication of which is controlled by the channel emulation module 20.

Referring briefly to FIG. 19 according to one aspect of the inventionthe RF combiners such as combiner 13 a each include self-terminatingcircuitry 335. The RF combiner essentially adds all signals coupled tothe RF combiner ports, and thus a non-terminated port would serve to addunpredictable artifacts to the combined signals. The RF combiner of thepresent invention includes a self terminating device 335 whichautomatically senses when a port is unconnected, and self-terminates tofurther isolate the combined signals from residual interference.

Referring back now to FIG. 2, as mentioned above, the operativecapabilities of devices 32 and 42 may be tested by emulating a changingphysical environment between the devices, and monitoring the devices'operation in the changing environment. The physical signals in thetransmission channel are modified by the channel emulation module inaccordance with the effect being emulated in the channel. The effectsthat are emulated by the channel emulation module are referred to hereinas ‘channel effects,’ and include but are not limited to multipathreflections, delay spread, angle of arrival, power angular spread, angleof departure, antenna spacing, uniform linear array for both TX and RXside, Doppler due to fluorescent light effects, Doppler from movingvehicle, Doppler from changing environments, path loss, shadow fadingeffects, reflections in clusters and external interference such as radarsignals, microwave oven emissions, phone transmission and other wirelesssignals or noise. The channel effects may be applied to datatransmission signals going through the channel emulation module or thechannel emulation module can serve as an interference generator with nosignals at its input. Several embodiments of channel emulation moduleswhich may be used to test various network topologies, protocols andcapabilities will be disclosed herein. The channel emulation modulesthat will be described below include an attenuation channel emulationmodule (for use in either Single Input, Single Output test environmentsor Multiple Input, Multiple Output test environments), a Cross-connectchannel emulation module, a Multiple Input, Multiple Output channelemulation module and a test module. However, it should be appreciatedthat the disclosed channel emulation modules are merely representative,and that any variety of other channel emulation modules that emulatechannel effects of various complexity may be substituted herein withoutaffecting the scope of the present invention. In addition, although thebelow channel emulation modules are shown and described as used with thebackplane, it should be appreciated that they may also be used in testenvironments without the backplane, by directly connecting test devicesand/or DUTs to RF connectors on the modules that mate with thebackplane.

Attenuation Channel Emulation Module

FIG. 2 illustrates one test environment 10 which includes a channelemulation module 20 used to test relationships between Single Input,Single Output (SISO) protocol devices. Such protocols include but arenot limited to any of the 802.11a, b or g protocols. FIG. 2 illustratesseveral access points 32, 34 and 36 which are coupled to several clientlaptops 42, 44 and 46 in a SISO test environment. Frequently it isdesirable to test the behavior of SISO devices in response to changingphysical environments caused, for example, by spatial movement of thedevices or the introduction interference into the network. Onerelatively straightforward embodiment of a channel emulation module 20-1is shown in more detail in FIG. 3. The channel emulation module 20-1 ofFIG. 3 includes a programmable attenuator, such as attenuator 210,disposed in each transmission path between the RF backplane and the RFports of the channel emulation module 20-1. In a SISO environment, thesignals 47 received from the backplane are associated with different RFdevices coupled to the RF combiners, and may utilize differenttransmission channels. In a test environment such as that illustrated inFIG. 2, each signal 41, 43 and 45 is also associated with a different RFdevice. Essentially, the programmable attenuator provides a means forcontrolling the attenuation of signals transferred between the differenttest devices. Appropriate control of each of the programmableattenuators may be used to emulate channel behavior in response todevice movement or interference.

Although the channel emulation module 20-1 has been shown and describedin a SISO environment, it can be appreciated that the same circuitry maybe used to provide low cost channel emulation in a Multiple Input,Multiple Output environment (MIMO). Although the circuitry 20-1 may havelimited use in testing MIMO radio performance, it may be helpful for thepurpose of testing Medium Access Control (MAC) and higher levelprotocols. In such a MIMO SUT, signals 47 are coupled to differentradios of a first MIMO DUT, signals 41, 43 and 45 are coupled todifferent radios of a second MIMO DUT, and the programmable attenuators210 are a low cost method of emulating channel behavior. More details ofMIMO operation and other embodiments of channel emulation modules thatmay be used to model MIMO multipath behavior will be described laterherein. Accordingly, the channel emulation circuitry of module 20-1 is alow cost module alternative that may be used to test a variety ofwireless protocols.

Cross-Connect Channel Emulation Module

While the channel emulation module 20-1 of FIG. 3 provides acost-effective method for simulation of movement between two SISOdevices, it is often desirable to simulate multi-dimensional spatialmovement between two devices to increase the robustness of roamingsimulation and test platforms. According to another aspect of theinvention, a cross connect channel emulation module may be used in aSISO environment to provide mesh attenuation connectivity between DUTsand test devices, thereby increasing the available network testtopologies as well as providing a foundation for simulation ofmulti-dimensional spatial movement between test devices. Exemplarycircuitry that may be included in such a cross-connect channel emulationmodule 20-2 is shown in FIG. 4.

The ability to provide mesh connectivity with attenuation allowsmulti-dimensional movement to be simulated with increased ease andaccuracy. As used herein, ‘mesh’ connectivity is meant to convey that apath is available from each test device to any other test device in thenetwork. Mesh connectivity is achieved through a combination of the RFcombiners and the backplane; each RF signal coupled to an individual RFcombiner is available to any column of the backplane matrix. The channelemulation module 20-2 may be included in the test system of FIG. 2 andcontrolled to emulate the movement of client devices 42, 44 and 46 tomonitor whether and when roaming between APs 32, 34 and 36 occurs.

FIG. 4 illustrates exemplary components that may be included in a crossconnect channel emulation module 20-2. The channel emulation module 20-2includes cross connect logic 50, which is disposed between the backplaneand RF ports 21 of the channel emulation module. Taps 51, 53 and 55 aredisposed between the cross connect logic 50 and the RF ports 21. Thetaps 51, 53 and 55 are also coupled to external traffic path 48 and amonitoring path 49. External traffic may be injected into thetransmission channel of the DUTs via the cross connect using theexternal traffic ports 48, for example to test the operation of devicesin the presence of a range of background traffic. The monitoring pathmay be coupled either to an external monitoring device (not shown inthis embodiment), or monitoring circuitry 54, 56 and 58 which isdisposed within the channel emulation module. The monitoring circuitryincludes Network Interface Cards (NICs) which interpret and analyzeexchanges between the devices in the transmission channel under test (inthis example, devices 42, 44 and 46) and the APs 32, 34 and 36.

FIGS. 4 and 5 have described a channel emulation module which uses asymmetric cross-connect. However, it should be understood that it is nota requirement that the cross-connect be symmetric, and in fact typicalnetwork topologies are often asymmetric. For example, an alternativeembodiment of a cross-connect channel emulation module 20-3 which may beused to emulate channel effects and connectivity in a network comprisedof 4 APs and 16 clients is shown in FIG. 6. Thus the cross-connectemulation module can be comprised of any N×M matrix, with attenuationcapabilities in each path. Each N×M cross connect would include N 1:Mcombiners coupled to backplane connectors, M N:1 combiners coupled totest ports of the emulation module, and N×M attenuators disposed betweenthe pairs of combiners.

According to one aspect of the invention, each attenuator in the crossconnect is independently programmable, for example, by a software testroutine operated by a test administrator. With such an arrangement, themovement of devices in the network may be emulated through appropriateadjustment of attenuation of the signals in the transmission channels.The full connectivity of the cross-connect permits simulation ofmulti-dimensional movement, thereby enabling a robust analysis ofroaming capabilities of the test devices. As has been shown anddescribed, the size and symmetry of the cross-connect is limited only bythe practical aspects of its insertion loss.

Multiple Input, Multiple Output Channel Emulation Module

Referring now to FIG. 7, a discussion of how the architecture of thepresent invention may be used to facilitate testing of a MIMOenvironment will now be shown and described. FIG. 7 illustrates thebasic phenomenon of multipath signal propagation. Because there areobstacles and reflectors in the wireless propagation channel, thetransmitted signal arrivals at the receiver from various directions overa multiplicity of paths. Multipath signals are therefore anunpredictable set of reflections and/or direct waves each with its owndegree of attenuation and delay.

The Institute of Electrical & Electronic Engineers (IEEE), aprofessional organization that helps set transmission system standards,is currently defining IEEE 802.11n™ which seeks to take advantage of themultipath phenomena. In an attempt to increase data throughput over thatwhich is available in 802.11a, b or g, 802.11n endorses using the signaldifferentiation provided by multipath phenomena to permit radiofrequency channel sharing by different data streams.

Multipath channel effects are also considered in transmit beam formingtechnology. In general, transmit beam forming uses antenna diversity toincrease communication quality; i.e., increase the transfer rate vs.range performance. In transmit beam forming, the same data is sent fromeach antenna, but with a phase/amplitude adjustment for each antenna,such that the signal quality is maximized at the receiver. Beam formingtechnology thus allows diversity and array gain to be achieved.Protocols and systems which utilize the multipath behavior oftransmission signals to their advantage (such as MIMO and beam formingtechnologies) are referred to herein after as MIMO protocols andsystems.

Referring now to FIG. 8, a MIMO access point 18 is coupled via RFcombiners 13 a-13 d to a MIMO channel emulation module 20-4. A MIMOclient device, such as laptop 24, is coupled to RF ports 21 of the MIMOchannel emulation module 20-4. The MIMO channel emulation module 20-4includes circuitry for emulating multipath channel effects in additionto other channel effects described above on each of the four signals inthe transmission channel shared by the access point 18 and the client24. The MIMO channel emulation may be implemented using circuitry havinga range of complexity; for example, from analog circuitry as shown inFIGS. 3 and 10 to digital signal processing devices, or a combinationthereof. Similar to the attenuation and cross-connect emulation modules,the MIMO emulation modules may be adapted to include integratedmonitoring functionality, or alternatively may be coupled to externalmonitoring devices.

In FIG. 8, AP 18 and client laptop 24 are each shown coupled to thebackplane and channel emulation module, respectively, via groups of fourcables (16 and 22). As mentioned above, the cables are coupled toantenna ports of the respective test devices, and are used to isolatethe signals from environmental interference as they are transferred fromthe test devices to the backplane/channel emulation module. In the MIMOtest environment, each RF signal is transmitted at the same frequency.The MIMO channel emulation module adds multipath channel effects, andmay be adapted to add one or more of the other channel effectsidentified above (delay spread, angle of arrival, power angular spread,angle of departure, antenna spacing etc.) to the signals. The channelemulation module controls the range of channel effects that are appliedto the multipath signals to test the range of operability of the MIMOdevices. The multipath effects that are applied to RF signals mayrepresent expected multipath behavior that is mathematically derivedbased on intuition and knowledge bases. Alternatively, the appliedmultipath effects may be obtained through measurement of actualmultipath transmission signal behavior using channel soundingtechniques. Channel sounding involves measuring RF signal path loss,delays, gains, phase shifts, etc., for an RF signal as it propagatesthrough a physical environment. The measured values can be saved andthen used as a channel profile for multipath emulation.

An exemplary embodiment of a MIMO channel emulation module 20-4 is shownin FIG. 9 to include an RF channel emulation component 95 disposedbetween backplane ports 91 and RF ports 21. The channel emulation moduleis also shown to advantageously include external traffic ports 97 andmonitoring ports 99. As mentioned above with regard to the cross-connectchannel emulation module, external traffic ports 97 may be used toinject background traffic into the transmission channel of the testdevices during test. Monitoring ports 99 may be coupled to an externalmonitor to permit monitoring of DUT behavior. Alternatively, monitoringblocks may be included within the channel emulation module in a mannersimilar to that illustrated in FIG. 4 of the cross-connect channelemulation module 20-2.

As mentioned above, emulation of multipath channel behavior may beperformed at a variety of complexities, and thus the RF channel emulatorcomponent 95 may comprise an associated variety of circuitry of variouscomplexities. FIG. 3, previously described, illustrated a low complexitycircuitry which may be included in the RF channel emulator component 95,wherein path loss is introduced on the signals using attenuators fortesting MAC and higher level protocols. Several other embodiments of RFchannel emulator components (195, 295, 395 and 495 in FIGS. 10-13respectively) which may be used to emulate multipath channel effects ata variety of complexities will now be shown and described. It should beappreciated that the disclosed embodiments are exemplary only, and thepresent invention encompasses any technique that may be substitutedherein for emulating multi path channel behavior. For example, channelemulator components 195 and 295 may replace the digital signal processor232 in FIGS. 12 and 13 to provide a bi-directional analog multipathemulator.

Referring now to FIG. 10, an illustration a first analog embodiment 195of the channel emulator component is shown. The channel emulator 195adds channel effects to the bi-directional signals exchanged between thebackplane and the RF ports 21 of the channel emulator module. Forpurposes only of facilitating understanding of the below description,the term ‘forward’ path shall be used to describe transmissions on apath originating at the backplane and ending at the RF Ports 21, and theterm ‘reverse path’ will be used to describe transmissions on a pathoriginating at the RF Ports 21 and directed at the backplane. Theillustrated embodiment shows circuitry that may be included to addmultipath to each one of N paths included in an N×M multipathtransmission channel under test. The RF channel emulator 195 includes asplitter/combiner 73 coupled to a plurality of delay lines 75 and aplurality of programmable attenuators 76. The delay lines are coupled toa splitter/combiner 77. The delay lines may be fixed or programmableanalog devices which add delay to N versions of the backplane signal.Signals from the backplane are differently delayed, attenuated andcombined at combiner 77 and forwarded to the RF ports 21. Signals fromRF ports 21 are similarly attenuated and delayed before being combinedat combiner 73 for forwarding to the backplane.

FIG. 11 illustrates an alternate embodiment of an analog channelemulator 295. The channel emulator 295 is shown to include a first setof combiners 200, a second set of combiners 208 and a set of attenuators204 and phase shifters 206 disposed there between. In addition, thechannel emulator is shown to include a multipath block 202. Eachmultipath block adds a potentially different multipath effect to thesignal, for example using circuitry similar to that of FIG. 10. Thesignals with multipath effects are forwarded to attenuators 204, whereprogrammable path loss may be applied to the signals according to adesired simulated behavior. The attenuated multipath-affected signalsmay be phase-adjusted by phase shifters 206, for example to testbehavior of ODFM transceivers or other types of transceivers and modems.

Because it is sometimes difficult to manipulate high frequency signals,it may be desirable to down-convert the RF signal to IF before using theanalog circuitry illustrated in FIGS. 10 and 11. Down-converted signalsmay be passed to analog circuitry, such as the channel emulatorcomponents 195 and 295 of FIGS. 10 and 11, or alternatively a digitalsignal processor such as DSP 232 shown in FIGS. 12 and 13. FIG. 12illustrates a uni-directional multipath emulation circuit 395, whileFIG. 13 illustrates a bi-direction multipath emulation circuit 495. BothMIMO emulators assume a four signal RF interface. Note that both FIGS.12 and 13 show reverse path circuitry which is not necessary if thebi-directional components 195 and 295 are substituted therein.

In FIG. 12, RF ports of the backplane are coupled to a circulator 220.RF signals from the backplane are forwarded to a Quad RF down converter230. The quad RF down converter converts the transmitted RF signals toIntermediate Frequency (IF) or baseband signals for processing by DSPengine 232. The DSP engine processes the input signals by applyingmultipath and other desired channel effects to each of the input signalsaccording to pre-programmed multipath channel profiles. The modifiedsignals are up-converted to RF, and attenuation is applied at attenuator225. The modified RF signal travels through a circulator 222 to the RFPorts 21.

The circulator also forwards RF signals received from RF Ports 21 to anyreverse path circuitry 236. As mentioned above, the reverse pathcircuitry may differ in the complexity from the circuitry used togenerate a forward path channel effect. For example, FIG. 12 illustratesthe inclusion of an amplifier in the reverse path to add gain to thesignal before forwarding to the backplane. Module controller 215controls the application of specific behavior models and attenuation tothe multipath signals.

Referring now to FIG. 13, an embodiment 495 of an RF channel emulator isshown wherein the reverse path circuitry is similar to the forward pathcircuitry described in FIG. 12. Thus in the reverse path a second RF toIF down converter 244 is provided for converting received RF signalsfrom circulator 222 to the mixer that down-converts the RF signal to IF,a DSP engine 242, an IF to RF up-converter 240 and programmableattenuation 245. DSP engine 242 and attenuation 245 are independentlyprogrammable and thereby allow different characteristics to be modeledon the return path.

One example of how the MIMO channel emulation modules may be used in atest environment is shown in FIG. 14. In FIG. 14 the test bed includestwo MIMO channel emulation modules 94 and 96. MIMO access point DUTantenna ports are coupled to the RF ports of each of the modules 94, and96, and a MIMO client DUT 78 is coupled to RF combiners 13 a-13 c. Avariety of channel effects, including multipath, path-loss, gain, etc.are applied to signals transferred between the DUTs in accordance with adesired test suite. For example, the test system may be used to monitora roaming behavior of the client in response to the emulation ofdifferent channel effects in the channel emulation modules.

Accordingly, a modular architecture has been shown and described whichmay be used to test a wide variety of network topologies and protocolsusing an arbitrary number of wireless devices. It is recognized thatthere is a cost associated with maintaining an inventory of wirelessproducts merely for purposes of testing; the cost of populating attestenvironment for use in verifying the operation of increasingly complexand capable devices in the presence of traffic from multiple devices canbecome prohibitive. In order to ensure that exhaustive and robusttesting can be provided for wireless devices of increasingly complexity,a test module of the present invention may be incorporated into themodular test environment.

The test module may be used in conjunction with a test chassis such asthose described in FIGS. 1-14. In addition, the test module can bedirectly coupled to DUTs without use of the chassis. Two such testenvironments 300 and 301 are shown in FIGS. 15 a and 15 b. The testmodule is shown stand-alone in both FIGS. 15 a and 15 b. FIG. 15 a showsa client DUT while 15 b shows an infrastructure system under test. FIG.15 a shows DUTs inside the shielded test head. FIG. 15 b does not showthe test head but assumes that DUTs are in test heads for isolation.

The test module 320 in one embodiment is a performance and protocol testplatform, programmable to test a variety of network protocols, includingbut not limited to 802.11a,b,g,n devices and systems. The test modulecombines client and AP emulation capabilities with a flexible protocoltest automation environment to enable thorough testing of device andsystem behavior for both SISO and MIMO SUTs in the presence of emulatednetwork traffic and fault conditions. Each test module advantageouslyincludes multiple network interfaces. Each network interface can beprogrammed to perform any one of a variety of functions, includingmonitoring and analyzing traffic on a channel, emulating one or more APor client traffic generating devices, or executing test scripts. Thetest module includes the ability to simultaneously generate traffic atboth RF and Ethernet interfaces.

FIG. 16 illustrates several components that may be included in anembodiment of a test module 320. The test module also includes a deviceinterface 311 and a chassis interface 309. Network interface blocks 304and 308 are used to transmit and receive traffic on the RF interfaces,while network interface blocks 306 and 310 transmit and receive trafficon the Ethernet interfaces of the network module. Thus each networkinterface block includes functionality for transmitting and receivingdata as either an 802.3 or 802.11 device. In addition, each wirelessnetwork interface advantageously includes circuitry for emulatingchannel effects on transmitted traffic, wherein the channel effects thatare added to the traffic include any of those described above, includingpath loss, gain, fading, angle of arrival, angle of departure, phaseadjustment and multipath channel effects.

A processor subsystem may be included within a network interface blockfor controlling that network interface, such as shown in networkinterface blocks 304 and 308. Alternatively, a processor subsystem maybe provided external to the network interface blocks, with the processorsubsystem controlling one or more network interfaces, each of which mayoperate using common or different network protocols. For example, inFIG. 16, processor subsystems in blocks 304 and 308 also control therespective Ethernet network interfaces 306 and 310. Thus, although theprocessor subsystem is shown integrated with the network interface inFIG. 16, embodiments of the present invention may use any combination ofmicroprocessors, located internally or externally with the networkinterfaces, each controlling one or more different interfaces, and thepresent invention is not limited in any manner to a particular placementof processing systems.

In the embodiment of FIG. 16, a Controller 322 is also provided. Varioustasks that are under taken during test, such as the generation andanalysis of traffic, may be apportioned between the controller and theprocessors as deemed appropriate by the test administrator. In oneembodiment, the Controller may also implement a state machineinterpreter that enables creation of software to implement protocol teststate machines. Switch matrix/combiner 302 controls the flow of trafficbetween the RF ports of the network module.

The components of the test module 320 may be used to support a varietyof test configurations. For example, they may be used to generatetraffic on multiple APs simultaneously while measuring aggregatethroughput of the system. Such a test configuration is shown in FIG. 17,where bi-directional multi-station traffic is used to measure throughputand capacity of the infrastructure. In one embodiment, the test moduleis capable of emulating traffic from up to 127 simulated clients on eachinterface, driving up to 8 APs simultaneously. Traffic is generated andanalyzed by all the 802.11 and gigabit Ethernet interfaces. The testconfiguration of FIG. 17 may also be used to emulate roaming of theclients from one AP to another. A fast roaming protocol, or one usingpre-authentication may be implemented as part of this roaming emulationto test the APs' ability to support fast roaming based on 802.11r.

In addition, traffic generated by the test module may incorporatemultiple network interface entities to create contention among emulateddevices for a realistic emulation of random network dynamics. Such aconfiguration is illustrated in FIG. 18, where traffic is exchanged withthe AP 340 over both Ethernet and RF interfaces. The test module isshown to include a combiner 365, which combines transmissions from thenetwork interfaces and the ports of the test module. The combination ofsignals will cause collisions on the multiple ports, thereby increasingthe reality of the test environment and thus the robustness of the test.The test module then measures performance parameters such as throughput,packet loss, delay, jitter, capacity, association performance, and otherproperties of the access point in the presence of network traffic.

In one embodiment, each network interface block may be dynamicallyprogrammed to assume one of at least 3 different modes—Client emulator,AP emulator, protocol analyzer, or other functions. FIG. 17 illustratesa test module embodiment wherein each network interface block includes aCPU, and wherein one of the N.I. blocks is programmed as a monitor, andis used to monitor transmissions in the transmission channel. A testmodule may be programmed to support either a SISO or MIMO environment.In one embodiment of client emulator mode, the network interface blockincludes capability for emulating up to 127 soft clients, for exampleincluding but not limited to data, voice or video devices, implementPower-save and Radio Resource Management (RRM), and support 802.11i,e,kprotocols. Each client and access point emulator can generate its owntransmit streams appropriate for the test at hand.

In addition to client and AP emulation, each network interface block inthe test module may be programmed using scripts to perform desiredprotocol, performance, interoperability or other testing. Programming ofthe interface blocks may be performed using a known scripting language,such as TCL. Certain interfaces may be dedicated to transmitting andothers to receiving to achieve maximum loading of the device under testwhile at the same time monitoring the progress of the test with no frameloss. Thus the particular functionality performed at any given time byeach network interface is a matter of test configuration, and will varydepending upon the particular protocols and capabilities being tested.

General test capabilities that may be included in each test moduleinclude the capability to test both client and AP devices, alone or as anetworked system, as well as functionality for analyzing test devicethroughput, capacity, roaming abilities, and range and protocolconformance. It may be desirable to include certain circuitry andprogram code in the test module in order to ensure that the general testcapabilities can be supported. For example, for some performance andbehavioral testing it is desirable to synchronize transmissions withother controls (e.g. attenuator settings). The controller 322 assistssynchronization circuitry with scheduling of multi-client transmissionsat desired resolutions. As described in the Mlinarsky patent, thesynchronization circuitry is advantageously disposed in the chassis of atest system to provide a sync signal to each component to resynchronizea clock internal to each system chassis to a specific, high precisionvalue.

For accurate throughput determinations, in order to measure how much ofthe offered test traffic is properly forwarded by a test device,statistical analysis should be performed at the same time as trafficgeneration. One way to implement concurrent statistical analysis is toanalyze traffic on the 802.11 and Ethernet interfaces simultaneouslywith traffic generation on both of these interfaces. The trafficanalysis and generation should support the fastest theoretical framerate and throughput.

For delay and jitter measurement, the test module should includecircuitry for inserting a time stamp into a transmit frame as part ofthe data field. Once the timestamp is inserted, the frame should to betransmitted onto the medium with a deterministic delay. Receive framesmust likewise be marked with a deterministic timestamp (e.g. in a bufferdescriptor). For transmissions where it is difficult to insert atime-stamp just before frame transmission due to queuing issues; eachtransmit frame may be recorded and matched with a receive frame tocalculate the delay through the DUT or SUT. In order to test the roamingperformance and functionality of the AP, the test module needs toimplement the client roaming algorithm that supports the lateststandards associated with fast roaming. In one embodiment, the testmodule is programmed using a command line or scripting interface.

Another feature of the test module is its ability to capture and decodetraffic on both 802.11 and 802.3 interfaces simultaneously, therebyallowing the test module to determine protocol compliance and measurevarious performance parameters (frame forwarding rate, roaming time,etc.).

Latency Measurement

According to another aspect of the invention, the test module mayinclude latency measurement circuitry to characterize latency of a DUT,where the DUT may be an AP or other infrastructure device. The latencyis defined as the time difference between when a frame is received atone network interface of a DUT and transmitted out of another networkinterface of the DUT.

Typical prior art latency measurement methods insert a first time stampinto a frame transmitted by a test module on a first network interfaceto the DUT. A second timestamp is added to the frame when it is receivedfrom the DUT at the test module. DUT latency is calculated using thedelta between the transmit and receive timestamps. However, access tothe network is non-deterministic; frame buffering and priorityscheduling cause indefinite delays between the time when the framesreceive their time stamps, and when transmission actually occurs.

The present invention overcomes the problems of the prior art bymonitoring frames transmitted from the network interface of the testmodule to the DUT, extracting sequence number from each transmittedframe, and storing, with the sequence numbers, a time stamp representingthe time the frame was actually transmitted from the network interface.Programmable logic, such as a Field Programmable Gate Array (FPGA)inserts time stamps into frames as they are received at the test module.The latency of the DUT can be computed by identifying transmitted andreceived frames having a common sequence number, and determining thedelta between the transmitted time stamp and the received time stamp.Because the time stamps reflect the time at which the frame was actuallytransmitted and received by the network interfaces, it removes thenondeterministic attributes from latency measurement, and provides a DUTlatency measurement having improved accuracy.

FIG. 20 is a functional flow diagram provided to illustrate exemplarysteps that may be performed during latency measurement, by exemplaryfunctional blocks of the test module. It should be noted that thefunctional blocks are representative only, and are used generally todescribe different components that may be included in a test module toperform latency measurement. Although many of the blocks are referred tobelow as processes, it should be understood that the functional blocksmay be implemented in software, hardware, or a combination thereof. Theprocess functionality may be implemented by any of the processors of thetest module, including processors both integrated with or external tothe network interfaces.

FIG. 20 illustrates the use of the functional components to measurelatency from a wireless interface to an Ethernet interface of a DUT 525.Packets are generated by a traffic generation process (not shown) andpassed to the Packet Transmit process 510 which places them in thewireless NIC transmit buffer descriptor ring 512. The Timestamp andSequence Number Extract process 514 is executed as part of a transmitcompletion interrupt. It performs the necessary operations to pair upthe sequence number contained in a field in all transmitted packets witha timestamp read from the Transmit Timestamp FIFO 532. The output ofthis process is placed into a queue 515 to wait for the same packet tobe received on the Ethernet interface.

A packet that was successfully transmitted by the wireless NIC, receivedby the AP and forwarded to the Ethernet side of the AP will be receivedby the Timestamp Insert FPGA 552. This FPGA 552 inserts a timestamp intoa field in the packet intended for this purpose. The TS Insert FPGA 552performs this operation on the fly and passes the resulting packet tothe test module Ethernet NIC 550.

A Packet Filter process 522 receives packets from the Ethernet NIC 550and determines whether the packet should be part of the latencycalculation. For example, Address Resolution Protocol (ARP) packet orsome other AP generated packet is would not be included into the latencymeasurement calculation.

After being approved by the Packet Filter process 522, the packet passesto another Timestamp and Sequence Number Extract process 524. Theprocess reads the receive timestamp and the sequence number from theiroffsets in the packet, and passes the information to the Sequence NumberMatch process 526.

The Sequence Number Match process searches for the received sequencenumber in the <Sequence Number, Timestamp> queue 515. When the correctentry is located, it is passed along with the received timestamp andsequence number to the Latency Compute process 533.

The Latency Compute process converts timestamps to the same units, ifnecessary, and computes the difference between receive and transmittimes to measure the latency. On completion, it stores the transmit timeand calculated latency in a data store for later processing or display.In this example of FIG. 20, since the delay measurement is performedbetween unlike network interfaces that use different time bases, thetransmit and receive timestamps cannot be correlated in astraight-forward manner. The transmit and receive timebases may driftwith respect to one another and this drift must be accounted for whendelay through the DUT is computed. To correlate the two timebases theactual drift between these timebases is periodically measured andcharacterized and taken into account when the time delay through the DUTis computed. Such a drift calibration process typically applies to thecases whenever the timestamps are provided by unlike network interfaces.

The delay measurement described here may advantageously be performedusing traffic that emulates a multitude of virtual clients since thedelay of an infrastructure device is a function of the number of activeclients that pass traffic through this device.

Referring now to FIG. 21, a data flow diagram is provided to illustratehow the latency measurement method and apparatus of the presentinvention may be used to measure latency from the Ethernet interface tothe RF interface through a DUT 625. As shown in FIG. 21, packets aregenerated by a traffic generation process (not shown) and passed to thePacket Transmit process 610 which places them in the Ethernet NICtransmit buffer descriptor ring 612 for transmission.

As the Ethernet NIC transmits a packet, the TS Insert FPGA 614 receivesthe packet and inserts a timestamp in the packet at the appropriateoffset from the end. It then retransmits the packet onto the Ethernetwhereupon it is received by the AP DUT 625. The AP 625 transmits it onits wireless interface and it is received by the test module wirelessnetwork interface card (NIC) 616.

The test module NIC may be any wireless NIC having the capability oftime stamping received frames. An example of one such NIC is the AR2312MAC/BB chip, manufactured by Atheros® Communications of Santa Clara,Calif. 2313, although other MAC NICs with similar capability may besubstituted. The Atheros MAC chip has the capability of timestamping areceived packet with one μs accuracy. The timestamp information isstored in the receive Buffer Descriptor (BD) ring 620 with the packet.

A Packet Filter process 622 examines packets on the receive BD ring 620and determines which packets are of interest in the latency computation.For instance, depending on the configuration of the MAC chip, variousframes irrelevant to the latency measurement may be filtered from thepacket stream to make sure only frames transmitted by the test moduleEthernet interface 612 are passed to the timestamp extract process 624.

Because the transmit timestamp is contained in the packet itself, whilethe receive timestamp is a field in the receive buffer descriptor, thetimestamp extract process 624 merely pairs up the timestamp informationand passes it to the latency compute process 626. Timestamps may becorrelated as described above.

The Latency Compute process 626 converts the timestamps to the sameunits and time scale, computes the difference between receive andtransmit times and stores the result.

The latency measurement methods and apparatus of the present inventionmay also be adapted to measure latency from an RF input to an RF outputof a DUT. In such a test scenario, each virtual client sends traffic toa fixed other virtual client. In other words, virtual client A sends tovirtual client B and vice versa. Thus the pool of virtual clients isapportioned into two equal groups. A virtual client in Group A sendspackets to a single virtual client in Group B; the Group B virtualclients do the same to the Group A virtual clients. Alternatively, eachvirtual client may send packets to every other virtual client, but forsimplicity of description, the first scenario will be described withrespect to FIG. 22.

As shown in FIG. 22, packets are generated by a traffic generationprocess (not shown) and passed to the Packet Transmit process 710 whichplaces them in the wireless NIC transmit buffer descriptor ring 714 fortransmission.

A Timestamp and Sequence Number Extract process 712 is executed as partof a transmit completion interrupt, and performs the necessaryoperations to pair up the sequence number contained in a field in alltransmitted packets with a timestamp read from the Transmit TimestampFIFO 716. The output of this process placed into a queue 724 to wait forthe same packet to be received on the wireless interface.

After having received the packet from the test module, the AP 725eventually retransmits it on its wireless interface (as is the procedurefor 802.11) and it is received by the test module wireless networkinterface card (NIC) 718, which places receive timestamp information inthe receive Buffer Descriptor (BD) ring 720 for the packet.

A Packet Filter process 722 examines packets on the receive BD ring anddetermines which packets are of interest in the latency computation,filtering out extraneous frames as described above. After being approvedby the Packet Filter process 722, the packet passes to another Timestampand Sequence Number Extract process 726. The process reads the receivetimestamp and the sequence number from their offsets in the packet, andpasses the information to the Sequence Number Match process 728.

The Sequence Number Match process searches for the received sequencenumber in the <Sequence Number, Timestamp> queue 724. When the correctentry is located, it is passed along with the received timestamp andsequence number to the Latency Compute process 730.

The Latency Compute process computes the difference between receive andtransmit times to measure the latency. On completion, it stores thetransmit time and calculated latency in data store for later processingor display.

Channel Effects Insertion

According to another aspect of the invention, the test module is able todistort signals in a transmission channel to emulate multipath and otherchannel effects. Thus, the network emulation may include circuitry suchas that disclosed above with regard to FIGS. 2-13. Channel models may beobtained using channel sounding techniques. Referring briefly to FIG. 7,in any environment, transmitted signals (for example from AP 70)encounter physical and environmental effects before reaching a receivingdevice, such as laptop 72. The physical and environmental effectsdistorts the signal(s) in the channel, adding channel effects whichreflect the air-link properties between the sending and receivingdevice. During DUT test, it is often desirable to analyze DUT operationin specific environments. Many existing systems use mathematicallymodeled simulations of air-link properties when analyzing DUT operation.The test administrator selects one of the mathematical models which mostclosely approximates the physical environment in which the DUT may beused, but usually the selection process involves trade-offs and as aresult the mathematical model merely approximates the physicalenvironment in which the DUT will be expected to perform.

The present invention facilitates air-link property modeling by allowingchannel effects to be recorded at a destination device for differentnetwork topologies and physical environments. The recorded signals canthen be played back by the network interface blocks during testing ofthe DUT, thereby permitting testing of the DUT in its intendedenvironment.

Noise and common interfering signals (caused, for example, by radar,microwaves, phones, Bluetooth® devices or thermal or impulse noise)could also be generated by DSP in the interface block as separatesignals without the transmit signal being present.

Accordingly, a modular wireless test architecture which may be used tocreate test environments capable of exercising a wide range ofprotocols, network topologies and device capabilities has been shown anddescribed. The modular architecture includes an RF backplane, at leastone RF combiner, and a channel emulation module which modifies RFsignals transmitted through the module in accordance with selectedchannel effects. Emulation modules capable of emulating differentchannel effects of varying complexity may be easily swapped into thechassis depending upon the devices and capabilities to be tested.External background traffic may be injected into the transmissionchannel via the emulation module, and monitoring circuitry capturessignal state in the channel for forwarding to internal or externaltraffic analyzers. As described above, a test module capable ofgenerating and/or analyzing both SISO and MIMO traffic from multipleclients and APs on multiple interfaces may be coupled directly to thebackplane to emulate additional network clients and access points, ormay alternatively be directly coupled to a DUT. Improved latencymeasurement techniques allow the test module to accurately measure thelatency between a variety of DUT network interfaces.

Having described an exemplary embodiment of the present invention, itwill be appreciated that various modifications may be made withoutdiverging from the spirit and scope of the invention. For example, aswireless protocols, topologies and capabilities continue to develop,channel emulation modules and test modules developed to test theevolving technology would be within the scope of the present invention.

The above specification has described present invention in terms offunctional blocks delineated in a manner to facilitate description.However, it should be noted that the invention may be implemented in avariety of arrangements, using hardware, software or a combinationthereof, and the present invention is not limited to the disclosedembodiment. While the invention is described through the above exemplaryembodiments, it will be understood by those of ordinary skill in the artthat modification to and variation of the illustrated embodiments may bemade without departing from the inventive concepts herein disclosed.Accordingly, the invention should not be viewed as limited except by thescope and spirit of the appended claims.

1. An apparatus for testing wireless devices comprising: a backplane; acombiner module, removably couplable to the backplane and including atleast one port for coupling a first device; and a channel emulationmodule removably couplable to the backplane and including at least oneemulator port for coupling a second device, the channel emulation modulefor modifying signals transferred in a transmission channel between thefirst device and the second RF device in accordance with a selectedchannel effect.
 2. The apparatus of claim 1, wherein the selectedchannel effect is comprised of one or more channel effects selected froma channel effect group including multipath reflection, delay spread,angle of arrival, power angular spread, angle of departure, antennaspacing, Doppler effects, path loss, fading and external interference.3. The apparatus of claim 1 further comprising N additional combiners,wherein N is greater than or equal to one.
 4. The apparatus of claim 3further comprising M additional channel emulation modules, wherein M isgreater than or equal to one.
 5. The apparatus of claim 1 furthercomprising M additional channel emulation modules, wherein M is greaterthan or equal to one.
 6. The apparatus of claim 1, wherein the combinerincludes a plurality of ports adapted to transmit and receive RF signalswhich are combined in the combiner, and wherein each port includesself-termination circuitry for sensing when the respective port of thecombiner is unconnected and for self-terminating the port in response tosensing that the port is unconnected.
 7. The apparatus of claim 1,wherein the channel emulation module comprises attenuation circuitry. 8.The apparatus of claim 1, wherein the channel emulation module comprisesdelay lines.
 9. The apparatus of claim 1, wherein the channel emulationmodule comprises a programmable digital signal processor.
 10. Theapparatus of claim 1, wherein the channel emulation module includes aplurality of emulator ports for coupling to a plurality of devices, andwherein the channel emulation module comprises cross-connect circuitryfor coupling a signal from the first device to any of the plurality ofemulator ports.
 11. The apparatus of claim 10, further comprising aplurality of combiners, each combiner including a plurality of combinerports, and wherein the cross connect circuitry couples any one of theplurality of combiner ports to any one of the plurality of emulatorports.
 12. The apparatus of claim 1 for use in testing N×M MultipleInput, Multiple Output devices, wherein the channel emulator modulecomprises M emulator ports and wherein the system further comprises: Ncombiners; and wherein the channel emulator includes circuitry foremulating multipath effects on a plurality of signals transferredbetween the N combiners and the M emulator ports.
 13. The apparatus ofclaim 12 wherein the channel emulator further emulates at least oneadditional channel effect on each of the plurality of signals.
 14. Theapparatus of claim 12 wherein channel effects are emulated by directlyapplying channel effects to RF signals in the transmission channel. 15.The apparatus of claim 12 wherein channel effects are emulated byconverting RF signals in the transmission channel to one of IF orbaseband signals, and directly applying channel effects to the IF orbaseband signals.
 16. The apparatus of claim 1, wherein the channelemulation module includes at least two interfaces, and wherein theapparatus further comprising a test module, couplable to the backplane,for generating traffic on one of the at least two interfaces.
 17. Theapparatus of claim 16, wherein the test module includes circuitry formonitoring traffic on the backplane.
 18. The apparatus of claim 16,wherein the test module includes circuitry for measuring delay andjitter.
 19. The apparatus of claim 1, wherein the channel emulationmodule comprises circulators.
 20. The apparatus of claim 1, wherein thechannel emulation module is bi-directional.
 21. The apparatus of claim 1wherein the signals are modified to emulate multi-dimensional spatialmovement between the first device and second device.
 22. The apparatusof claim 21 wherein the modification of signals to emulatemulti-dimensional spatial movement is used to test the roamingcapabilities of the first and second devices.
 23. The apparatus of claim21 wherein traffic is forwarded between the first and second devices andwherein performance characteristics of the devices are evaluated asmulti-dimensional spatial movement is emulated.
 24. The apparatus ofclaim 23, wherein the traffic is video traffic.
 25. The apparatus ofclaim 23 wherein the traffic is voice traffic.
 26. The apparatus ofclaim 23, wherein the performance characteristics include quality ofservice characteristics such as throughput, packet loss, delay, jitter.27. The apparatus of claim 21 wherein the spatial movement is emulatedby controlling programmable attenuators in the channel emulation moduleusing pre-recorded signal information associated with themulti-dimensional spatial movement.